The present invention relates to a method for fabricating a nitride semiconductor in which the density of a p-type dopant is positively increased, a method for fabricating a nitride semiconductor device, and a nitride semiconductor device fabricated by this method.
Prior art techniques of doping a nitride semiconductor device with a p-type dopant, in particular, magnesium (Mg) will be described.
In the first prior art (Japanese Journal of Applied Physics, 38, L1012, 1999), a superlattice (SL) layer having a cycle of 36 nm is disclosed for use as a p-type cladding layer paired with an n-type cladding layer to sandwich an active layer in the direction vertical to a substrate and confine light generated from the active layer. Each cycle of the superlattice layer is composed of an aluminum gallium nitride (Al0.15Ga0.85N) layer having a thickness of 24 nm and a gallium nitride (GaN) layer having a thickness of about 12 nm, for example. In this disclosure, the cycle of the superlattice layer is in the range of 9 nm to 100 nm.
Doping of the p-type cladding layer with magnesium (Mg) is performed uniformly over the entire superlattice layer. There is another disclosure reporting doping of either the AlGaN layers or the GaN layers. In either case, doping is uniform in each layer of the AlGaN layers and/or the GaN layers. This p-type cladding layer is formed on a substrate in a following manner. That is, using decompressed metal-organic vapor phase epitaxy (MOVPE) under a growth pressure of 300 Torr (1 Torr=133.322 Pa), a buffer layer made of aluminum nitride (AlN) is grown on a sapphire substrate of which the principal plane is the C plane at a substrate temperature of 400xc2x0C., and subsequently an undoped gallium nitride (GaN) layer having a thickness of 1 xcexcm is grown on the buffer layer at a raised temperature. The substrate temperature is then raised to 1010xc2x0 C., and the superlattice layer is grown.
By adopting the above method, strain occurs between the AlGaN layer and the GaN layer, causing generation of an internal electric field. This makes the acceptor level of Mg shallow and thus improves the activation yield of the acceptor. Therefore, the p-type carrier density (hole density) increases, and this advantageously reduces the threshold current of the laser device.
In the second prior art (Japanese Laid-Open Patent Publication No. 8-97471), disclosed is a first contact layer made of highly doped p-type GaN that is in contact with an electrode made of nickel (Ni). The first contact layer has a thickness of 50 nm and a Mg density in the range of 1xc3x971020 cmxe2x88x923 to 1xc3x971021 cmxe2x88x923. This prior art discusses that with this construction, the contact resistance can be reduced, and also the operating voltage of the device can be lowered by attaining a high carrier density.
In the second prior art, if the first contact layer is doped with Mg at an excessively high density, the hole density contrarily becomes low. To overcome this problem, a second contact layer made of p-type GaN having a Mg density lower than the first contact layer is formed on the surface of the first contact layer opposite to the electrode. According to this prior art, the second contact layer is desirably doped with Mg at a density in the range of 1xc3x971019 cmxe2x88x923 to 5xc3x971020 cmxe2x88x923 for the purpose of increasing the hole density.
The prior art techniques described above have the following problems. In the first prior art, the superlattice structure of the p-type cladding layer is yet insufficient in attaining low resistance. In the second prior art, although the upper portion of the p-type contact layer is doped with the p-type dopant at a high density, this contrarily decreases the hole density.
In addition, the conventional doping techniques find difficulty in providing a steep impurity profile. In particular, when a p-type cap layer is formed on an active layer, for example, an especially steep impurity profile is required for suppression of diffusion of a p-type dopant to the active layer.
An object of the present invention is attaining low resistance of a nitride semiconductor by increasing the p-type impurity density of the nitride semiconductor without increasing the doping amount and also providing a steep p-type impurity profile.
To attain the above object, according to the present invention, a first semiconductor layer made of a group III nitride is formed in contact with a second semiconductor layer made of a group III nitride different in composition from that of the first semiconductor layer, so that the density of a p-type dopant locally increases in an area near the heterojunction interface between the first and second semiconductor layers, that is, so that segregation of the p-type dopant occurs.
The method for fabricating a nitride semiconductor of the present invention includes the steps of: (1) growing a first semiconductor layer made of a first group III nitride over a substrate by supplying a first group III source and a group V source containing nitrogen; and (2) growing a second semiconductor layer made of a second group III nitride on the first semiconductor layer by supplying a second group III source and a group V source containing nitrogen, wherein at least one of the steps (1) and (2) includes the step of supplying a p-type dopant over the substrate, and an area near the interface between the first semiconductor layer and the second semiconductor layer is grown so that the density of the p-type dopant locally increases.
According to the method for fabricating a nitride semiconductor of the present invention, by forming a layered structure of the first semiconductor layer and the second semiconductor layer, the density of the p-type dopant in the layered structure increases compared with the conventional case. This makes it possible to attain low resistance, and also attain a steep p-type impurity profile in which only the layered structure has a high impurity density.
In the method for fabricating a nitride semiconductor of the present invention, preferably, the first group III source contains gallium, and the second group III source contains aluminum or indium. This further ensures increase in the density of the p-type dopant in the layered structure compared with the conventional case.
In the method for fabricating a nitride semiconductor of the present invention, preferably, the first group III source mainly contains gallium, and the second group III source contains gallium and either one of aluminum and indium. This further ensures increase in the density of the p-type dopant in the layered structure compared with the conventional case.
In the method for fabricating a nitride semiconductor of the present invention, when both the step (1) and the step (2) include the step of supplying a p-type dopant, the supply amount of the p-type dopant is preferably roughly the same in the two steps. Even in this uniform doping with the p-type dopant, it is possible to locally increase the density of the p-type dopant in an area near the interface between the first and second semiconductor layers.
In the method for fabricating a nitride semiconductor of the present invention, the supply amount of the p-type dopant is preferably different between the step (1) and the step (2). Even in this selective doping with the p-type dopant, it is possible to locally increase the density of the p-type dopant in an area near the interface between the first and second semiconductor layers.
In the method for fabricating a nitride semiconductor of the present invention, when the p-type dopant is supplied during the growth of the first semiconductor layer, the supply of the p-type dopant is preferably started ahead of the growth of the first semiconductor layer. Likewise, when the p-type dopant is supplied during the growth of the second semiconductor layer, the supply of the p-type dopant is preferably started ahead of the growth of the second semiconductor layer. By this advanced supply of the p-type dopant, the p-type dopant that is to be introduced into the semiconductor layer under growth can reach the growth surface of the semiconductor layer without delay. This ensures attainment of a steep impurity profile.
In the method for fabricating a nitride semiconductor of the present invention, the peak of the density of the p-type dopant is preferably located in the second semiconductor layer.
In the method for fabricating a nitride semiconductor of the present invention, preferably, the second group III source contains a plurality of group III elements, and the peak position of the density of the element having a smaller mole fraction among the plurality of group III elements is different from the peak position of the density of the p-type dopant.
In the method for fabricating a nitride semiconductor of the present invention, the density of the p-type dopant is preferably about 3xc3x971019 cmxe2x88x923 or less. This makes it possible to increase the effective acceptor density of the p-type dopant.
In the method for fabricating a nitride semiconductor of the present invention, the thickness of the second semiconductor layer is preferably about 1.5 nm or more. By this setting, it is possible to locate the peak position of the p-type dopant inside the second semiconductor layer.
The method for fabricating a nitride semiconductor device of the present invention includes the steps of: (1) growing an active layer made of a first nitride semiconductor on a substrate; (2) growing a p-type cap layer made of a second nitride semiconductor on the active layer for protecting the active layer; (3) growing a p-type cladding layer made of a third nitride semiconductor on the p-type cap layer; and (4) growing a p-type contact layer made of a fourth nitride semiconductor on the p-type cladding layer, wherein at least one of the steps (2), (3), and (4) includes the steps of: growing one layer made of a first group III nitride by supplying a first group III source and a group V source containing nitrogen; and growing another layer made of a second group III nitride on the one layer by supplying a second group III source and a group V source containing nitrogen, wherein at least one of the step of growing one layer and the step of growing another layer includes the step of supplying a p-type dopant to the substrate, and an area near the interface between the one layer and the another layer is grown so that the density of the p-type dopant locally increases.
According to the method for fabricating a nitride semiconductor device of the present invention, the method for fabricating a nitride semiconductor of the present invention is employed for formation of at least one of the p-type cap layer, the p-type cladding layer, and the p-type contact layer of the nitride semiconductor device. This makes it possible to attain low resistance and a steep impurity profile for at least one of the p-type cap layer, the p-type cladding layer, and the p-type contact layer.
In the method for fabricating a nitride semiconductor device of the present invention, preferably, the first group III source contains gallium, and the second group III source contains aluminum or indium.
In the method for fabricating a nitride semiconductor device of the present invention, the supply of the p-type dopant is preferably started before the growth of the one layer or the another layer.
In the method for fabricating a nitride semiconductor device of the present invention, the density of the p-type dopant in the p-type cap layer or the p-type cladding layer is preferably about 3xc3x971019 cmxe2x88x923 or less.
In the method for fabricating a nitride semiconductor device of the present invention, the thickness of the another layer is preferably about 1.5 nm or more.
In the method for fabricating a nitride semiconductor device of the present invention, preferably, the p-type contact layer contains indium, and the density of the p-type dopant in the p-type contact layer gradually decreases as the position is deeper from the surface of the p-type contact layer, and is about 3xc3x971019 cmxe2x88x923 or more at a position about 10 nm deep from the top surface.
The nitride semiconductor device of the present invention includes: an active layer made of a first nitride semiconductor formed on a substrate; a p-type cap layer made of a second nitride semiconductor formed on the active layer; a p-type cladding layer made of a third nitride semiconductor formed on the p-type cap layer; and a p-type contact layer made of a fourth nitride semiconductor formed on the p-type cladding layer, wherein at least one of the p-type cap layer, the p-type cladding layer, and the p-type contact layer has a multilayer structure of one layer made of a first group III nitride and another layer made of a second group III nitride different from the first group III nitride formed on the one layer, and the density of the p-type dopant locally increases in an area of the another layer near the interface with the one layer.
In the nitride semiconductor device of the present invention, preferably, the first group III source contains gallium, and the second group III source contains aluminum or indium. This makes it possible to attain a semiconductor layer device capable of oscillating short-wavelength laser light such as violet light.
In the nitride semiconductor device of the present invention, the density of the p-type dopant in the p-type cap layer or the p-type cladding layer is preferably about 3xc3x971019 cmxe2x88x923 or less.
In the nitride semiconductor device of the present invention, the thickness of the another semiconductor layer is preferably about 1.5 nm or more.
In the nitride semiconductor device of the present invention, preferably, the another layer of the p-type contact layer contains indium, and the density of the p-type dopant in the p-type contact layer gradually decreases as the position is deeper from the surface of the p-type contact layer, and is about 3xc3x971019 cmxe2x88x923 or more at a position about 10 nm deep from the top surface.
The 61st Annual Meeting Digest 3a-Y-30, September, 2000 of The Japan Society of Applied Physics describes a strained-layer superlattice (SLS) structure with each cycle of 5 nm composed of an Al0.16Ga0.84N layer having a thickness of 2.5 nm and a GaN layer having a thickness of 2.5 nm. Both the AlGaN layers and the GaN layers are doped with Mg, a p-type dopant, uniformly at a density of 7xc3x971019 cmxe2x88x923. From analysis by secondary ion mass spectrometry (SIMS), a phenomenon that Mg is selectively incorporated in the AlGaN layers as the barrier layers is observed although the cycle is extremely short. In this case, however, because the resolution in the substrate depth direction is not sufficient, it is impossible to determine whether or not segregation of Mg has occurred in areas near the heterojunction interfaces.